1. Field
The disclosed concept relates generally to electrical switching apparatus and, more particularly, to circuit interrupters. The disclosed concept further relates to circuit interrupters providing ground fault protection. The disclosed concept also pertains to systems providing ground fault protection.
2. Background Information
One type of electrical switching apparatus is a circuit interrupter. Circuit interrupters, such as for example and without limitation, circuit breakers, are typically used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition, a short circuit, or another fault condition, such as an arc fault or a ground fault. Circuit breakers typically include separable contacts. The separable contacts may be operated either manually by way of an operator handle or automatically in response to a detected fault condition. Typically, such circuit breakers include an operating mechanism, which is designed to rapidly open and close the separable contacts, and a trip mechanism, such as a trip unit, which senses a number of fault conditions to trip the breaker automatically. Upon sensing a fault condition, the trip unit trips the operating mechanism to a trip state, which moves the separable contacts to their open position.
FIG. 1 is a circuit diagram of a system including a prior circuit interrupter 1. The circuit interrupter 1 is electrically connected to a power source 2 and a neutral 3 on its upstream side and a load circuit 4 on its downstream side. The circuit interrupter 1 includes first and second electrical conductors 5,6 electrically connected to outputs of the power source 2. The circuit interrupter 1 also includes separable contacts 7 and an operating mechanism 8 which is configured to open and close the separable contacts 7. The circuit interrupter 1 further includes a trip circuit which is electrically connected between the first and second electrical conductors 5,6. The trip circuit includes a trip actuator 9 which cooperates with the operating mechanism 8 to trip open the separable contacts 7. The trip actuator 9 includes a trip coil 10 which initiates tripping of the separable contacts when sufficient current is passed therethrough. The trip actuator 9 also includes a silicon controlled rectifier (SCR) 11 which turns on and off to control whether current passes through the trip coil 10.
The circuit interrupter 1 includes a ground fault protection circuit which detects a ground fault condition. A ground fault condition can arise when current flows to ground 20 on the downstream side of the circuit interrupter 1. A ground fault load 21 (shown in phantom line drawing) represents an impedance between the downstream side of the circuit interrupter 1 and ground 20. The ground fault condition can be detected based on a difference between currents in the first and second electrical conductors 5,6 inside the circuit interrupter 1. The ground fault protection circuit includes a current transformer 12 that senses a ground fault current as a difference between the current passing through the first electrical conductor 5 and the second electrical conductor 6. The ground fault protection circuit also includes an amplifier circuit 13 that converts the sensed ground fault current to a voltage and outputs it to a processor 14.
The processor 14 is powered by a power supply 15 which converts alternating current power from the first and second electrical conductors 5,6 to direct current power. The processor 14 determines whether a ground fault exists based on the converted ground fault current from the amplifier circuit 13. When the processor 14 determines that a ground fault condition exists, the processor 14 outputs a signal to the gate of the SCR 11 to turn on the SCR 11, thus allowing current to pass through the trip coil 10 and cause the separable contacts 7 to trip open.
UL943 is a standard for ground fault circuit interrupters. According to UL943, at a ground fault current of 264 mA, the circuit interrupter 1 should trip the power circuit to the load 4 within 25 ms. However, during a powering up period of the processor 14 and power supply 15, the response time of the processor 14 to the ground fault current may not be fast enough to meet this requirement of UL943.
There is room for improvement in circuit interrupters.